The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base stations at a lower price and also wish to reduce the running costs of the base station. Active antenna arrangements with Doherty Amplifiers have proven to meet these goals.
The Doherty amplifier is first known from U.S. Pat. No. 2,658,959 as an efficiency improved amplifier arrangement made of vacuum tubes for modulated signals. Since then the name Doherty amplifier has been recognized in the industry to refer to two parallel amplifier stages (vacuum tubes which were subsequently substituted by transistors), whereby a first amplifier stage operates in class AB mode and a second amplifier stage operates in class C mode. Usually the first stage is biased in such a way that it linearly amplifies the input signal of the first stage from zero excitation to carrier level. The first amplifier stage therefore is also called main stage. The second amplifier stage is biased in such a way that it amplifies signals above a certain threshold, i.e. input signals above the carrier level. Therefore it is usually called peak amplifier stage. In order to improve load balancing, the input signals of both amplifiers are shifted in phase so that the phase difference between the input signals of both amplifier stages is 90 degrees apart. In this way the phase of the output signals of the main stage and the peak stage are also 90 degrees apart. In order to form the output signal of the Doherty amplifier the phase shifted output signals are recombined in-phase.
In the original U.S. Pat. No. 2,658,959 the phase shifting between the input signal of the main stage and the input signal of the peak stage was achieved by a LC-voltage divider between the input of the amplifier arrangement and the input of the main stage; and a CL-voltage divider between the input of the amplifier arrangement and the input of the peak amplifier stage. As a result of this arrangement one input signal was retarded by 45 degrees in relation to the input signal of the amplifier arrangement. In contrast hereto the input signal of the other amplifier was 45 degrees advanced in relation to the input signal. The overall effect was to make both amplifiers work at a phase difference of 90 degrees.
In the paper “A New High-Efficiency Power Amplifier for Modulated Waves”, Bell Telephone System Technical Publications B-931 in 1936, Doherty also describes the use of 90 degree networks. FIG. 9 in this paper depicts two different applications. In the first application the input signal of the Doherty amplifier is passed through a −90 degree network before it is fed to the input of the first amplifier stage, whereas the input signal of the Doherty amplifier is fed directly to the input of the second amplifier stage, without applying any phase changes. A second −90 degree network at the output of the second amplifier stage retards the output of the second amplifier stage, so that the output signal of first amplifier stage and the output signal of the second amplifier stage after the second −90 degree network are in-phase and can be re-combined. A second application depicted in the said figure shows how to use a negative shifting 90 degree network at the input and a positive 90 degree shifting network at the output of the same amplifier stage. By this the phase difference within the same stage is compensated and the signals of the first and the second stage can be re-combined in-phase.
The stages of a Doherty amplifier may be formed by bipolar transistors or field effect transistors (FET). US Patent Application Publication 2009/0179702 A1 shows a Doherty amplifier arrangement comprising first and second bipolar transistors as well as first and second field effect transistors.